Frequency converter



Feb. 22, 1966 s. KRASNOW 2 9 FREQUENCY CONVERTER Original Filed Oct. 8,1956 2 Sheets-Sheet V 'o zo Me 9, 5 i k I 7 INVENTOR.

BY SHELLEY KRASNOW I Feb. 22, 1966 s. KRSNO w 3,237,084

FREQUENCY CONVERTER Original Filed Oct. 8, 1956 2 Sheets-Sheet 2 I mgINVENTOR United States Patent 3,237,084 FREQUENCY CONVERTER ShelleyKrasnow, Fairfax, Va.

(315 Tudor Lane, Manassas, Va.)

Original application Oct. 8, 1956, Ser. No. 614,599, now Patent No.3,083,311, dated Mar. 26, 1963. Divided and this application Dec. 21,1962, Ser. No. 246,878

6 Claims. (Cl. 321-63) This is a division of application Serial No.614,599, filed October 8, 1956 and now 'Patent No. 3,083,3l1.

This invention -relates to dynamo-electric machines and the -circuits tobe used therewith. There will be described herein a particular type of`structure and a circuit to be used in conjunction with such structurefor high frequency fluorescent lighting.

Use of frequencies above 60 cycles with the conventional type offluorescent lamp 'has been found to` yield markedly superior results inoperation and in life of the lamps. Frequencies of 360 and 400 cycleshave been used for this purpose. However, as is recognized in this art,frequencies above 400 cycles yield even superior results. Although theprinciples of -constru-ction of the machine to be described herein andof the circuit for use the-rewith, will not be altered by the frequency,the particular example chosen for illustration herein will be designedto operate at 840 cycles.

As will be obvious to those skilled in this art, many of the features ofConstruction of the dynamo-electric machine will be appropriate andapplicable to other similar structures such as motors, magnetos,electrically or magnetically ope-rated clutches, servo-systems, etc.

It is one purpose of the invention to provide a compact unitaryeflicient structure for the 'COIIVCISOII of low frequencies such as 25,50 or 60 cycles to higher -frequencies.

It is a further purpose of the invention to provide a highly efficientmagnetic structure for the rotor of a dynamo-ele-ctric machine and toprovide such structure and associated stator to deliver a wave form witha high per- -centage of harmonic components, suitable for use by highfrequency fiuorescent lighting systems.

It is a further purpose of the invention to provide a inechanicallystrong rotor structure designed to withstand high rotational speeds.

It is a further purpose of the invention to provide a stator structurefor use in conjunction with the improved rotor described in order toprovide a convenient means of obtaining the desired frequency,harmonics, and voltage.

It is a further purpose of the invention to provide an armature windingin which the voltage stress between adjacent conductors used thereinwill be reduced to a minimum and will thus avoid danger of breakdown.

It is a further purpose of the invention to provide an external circuitespecially matched to the properties of the converter to be described soas to obtain superior results for the converter and circuit combnation.

It is a further purpose of the invention to provide a circuit associatedwith the converter to be described, which will maintain a relativelyconstant voltage for all conditions of load, without the use of anyregulating equipment.

It is a further purpose of the invention to permit the use of aconverter of the general type described in conjunction with a load ofhigh leading power factor, without the necessity for complex regulatingdevices.

These and other purposes of the invention will be apparent from thespecications taken in conjunction with the drawings in which:

FIGURE 1 shows a cross-sectional View of a complete motor-generatorfrequency converter according to the inventon.

3,237,084 Patented Feb. 22, 1966 FIGURE 2 shows an end view of theconverter shown in FIGURE 1 viewed from the generator end.

FIGURE 3 shows a cross-sectional view of the rotor and stator of thegenerator shown in FIGURE 2, taken across the plane 2 2.

FIGURE 4 shows a partial cross-section of a portion of the rotor shownin FIGURE 3 taken across the plane 3 3.

FIGURE 5 shows a schematic view of the armature windings utilized in thegenerator shown in FIGURE 2.

FIGURE 6 shows the connection diagram for the interconnection of thearmature coils shown in FIGURE 5.

FIGURE 7 shows a wiring diagram of the armature as presented to theexternal circuit.

FIGURE 8 shows a typical external circuit including a fluorescent lampand its auxiliaries, suitable for connection to .the generator shown inFIGURE 2.

FIGURE 9 shows the method of magnetization of the magnets shown in theportion of the rotor structure pictured in F IGURE 4.

FIGURE 10 shows the magnetic lines of force due to the magnets in FIGURE4.

FIGURE 11 shows an oscillogram of the wave form obtamed with thegenerator shown in FIGURE 2.

Referring now particularly to FIGURE l, 1 shows a completemotor-generator assemblage. This consists of a two-part housing, element2 being the motor portion and 3 the generator portion. Within portion 2is a motor stator 4 and free to rotate therein, a motor rotor 5.

The motor may be of any of the types familiar in the Thus -it may be adirect current type, single phase alternating current, 2 or 3 phasealternating current, etc. A preferred type for most applications will=`be the conventional squirrel-cage type of 3-phase induction motor.

Within housing 3 is a generator stator 6 and free to rotate therein,generator rotor 7. A shaft extends through and is r igidly attached torotors 5 and 7 respectively and rests in bearings 9 and 9 respectively.Connection is made to the motor for energisation thereof through leads10. The output of the generator portion is delivered through leads 1 1.The energisat ion of motor 2 will cause rotation of shaft 8 andconsequent rotation of rotor 7 within stator 6. This will generateelectric power to be delivered through leads l l.

The detail of .the generator portion is further shown in FIGURE 2.Although many of the features of the invention may be atta-ined withgenerators of electromagnetically excited types, the particular type tobe described herein will be one utilizing permanent magnets forexoitation. The rotor 7 of this machine is constructed about awheel-like member with an outer peniphery of a ferromagnetic material. Apreferred form consists of a hub portion 13 which fits upon shaft 8,attached by means of a web or spokes 16 to rim 17. The material of rim17 may be mild or wrought steel, wrought iron, cast steel or a grade ofmalleable iron of the general type known as Magtiz." The portions 13 and16 need not be of the same material, although it has been foundconvenient to construct the `wheel portion 7 of a single casting orforging of the shape shown. Mounted upon machined surfa'ces 18 on rim 17-are a plurality of magnets 19. In the type of Construction shown, eachmagnet defines a pole. The machine may therefore have a minimum numberof 2 magnets or may have any desired larger number, depending upon thefrequency desired and the rotational speed to be used. In the particularexample shown, 28 magnets are utilized with a rotational speed of 3600r.p.m., yielding 840 cycles.

The magnets 19 are of a form diiferent than those heretofore used, beingof trusto-pyramidal form with the br-o-adest portion mounted facing thecenter of the rotor 7. The reasons for the selection of the formdescribed wil l be trea ted further herein.

Resting upon magnets 19 are pole pieces of ferromagnetic material 20.These lare generally of highly perme- -able material such as siliconsteel and may be made of a solid piece of steel or of a series ofsilicon steel laminations resting edgewise upon the magnets and fastenedtogether so as to constitute an integral pole piece. The pole pieces 20are preferably made with sharply defined corners 223 and a lso withSlots 24 cut into the surface of the pole face. The purpose of the sharpcorner 2 3 and the slot 24 is to introduce harmonics into the generatedwave.

After preliminary assembly of elements 16, 19 and 20, molten aluminumalloy is cast around the peripheral part of the structure and allowed tosolidify; After sol idification, the rotor may be finally machined todimension and provided with balancing holes 14, between each pair ofmagnets 19, 19.

The magnets 19 may be made of cast Alnico magnet material, coba ltsteel, or any of the other commonly available permanent magnetmaterials.

'In the specific example shown a strongly magnetc permanent magnetmaterial is utilized. However, with a leading power factor load, such asmet with in a fluorescent lamp" and capacitor ballast arrangement, aweakly magnetic permanent magnet material or even ordinary steel oriron, may be used for magnets 19.

`Balancing of the rotor 7 `to correct the inevitable misdistribution ofweight therein and to prevent vibration, is eifected by piacing heavyslugs such as lead within the balancing holes 14.

The conventional methods of balancing by drilling out material are notsuitable for ose with this structure, since the aluminum being a lightmaterial, will 'require removal of an undue amount in order to effectbalance. Removal of such amoun ts will seriously weaken the rotorstructure since its" strength is greatly dependent upon the strength ofthe cast aluminum The use of the tapered magnet-s 19 in the manner shownpermits the casting or dr illing of larger balancing holes `1 4, withoutweakeningor endangering the strengt h of the cast aluminum portion 12.Balancing slugs may be placed toward either edge of the rotor 7 in orderto eifect dynamic `as well as static balance.

The taper of the magnets as shown has been found to yield superiorresults in many respects. It has been common in the art to provideindividual 'magnets for the purpose described of para llel form such asrectangular blocks or with a taper facing inwardly and having the largefbase portion at the .periphery, rather than toward the mountingstructure of the rotor. In contradis tinction, in the presentinventio'n, the broader portion of the magnet rests against the flange.One conseque'nce of the use of the tapered magnet has already beenmentioned, namely making possible the enlargement of balanci'ng hole 14.Another advantage resides in the hysical str'engthening of the rotor.After the casting of the moiten aluminum, during solidification thereofthe atuminum will shrnk. This will cause i-t to exert a compressivstress inward upon the elements cast therein. Since the magnets 'havetapered sides, the result of this 'Stress will be to press the magnetsmore tightly against the rim 17 :and thus hold the magnet in tight magnetic contact with said rim 17. The centrifugal Stress on the magnetdue to rotation of the rotor will be resiste'd by the aluminum casttherearound. In eifect, the aluminum will have -a tapered pocket inwhich each magnet rests and the centrif ugal force will have the eifectof pressing the magnet in up against the tapered pocket, the i atterbeing a particularly effective form for resisting such Stress.

A still further advantage of the tapered for-m of the magnet will beseen to follow from the magnetiZing process that 'is utilized formagnetizi ng magnets `19 after assembly in the rotor 7. The magnetizingfixture consists of a removabl horse shoe shaped member 21, the faces ofwhich are machined to be in close contact with the pole faces 20-20. Thecoii 2 2 serves to energize horseshoe member 221 and to causeestablishment of a magnetic flux therein. A high value of the directcurrent is passed through coil 22 and a high value of magn etic flux isthe-reby set up in the magnet-ic circuit composed of member 2:1. polefaces' 20. magnets 19 and rim 17.

It will be seen that in the magnetizing process, useful lines of forcesuch as (a) will pass through the entire structure, while non-useful orparasitic leakage lines of force (b) will pass through the air betweenmagnets 19, 19. These will not serve to magnetize the magnet and in anyevent will not magnetize it in the direction desred. The closer theflanks of the magnets 19, 19, the greater will be the leakage and thegreater robbng of the useful flux utilized for magnetization. Thetapered form of the magnet reduces the leakage flux by increasing thedistance between the sides of the magnets. Magnetization is thereforeimproved.

Superior performance during operation also results from the use of thetapered magnets. Thus, as shown in FIGURE 10, the flux produced by themagnet passes through pole pieces 20 and into the armature 15. However,parasitic lines of leakage flux also exist between the magnets and serveto reduce the total amount of the useful working flux; By tapering themagnet distance between the faces is increased and the leakage factorreduced.

A still further advantage of the tapered form, where very hard magneticmaterials of the type of Alnico must be utilized, and such materials arecast, is that the tapered form makes it much easier to mold and draw thecastings from the mold.

In operation of high frequency fluorescent lighting systems, it has beenfound that the performance improves the higher the frequency, up tofrequencies of the order of 20,000 cycles. However, as one proceedsabove 400 cycl-es, the obtaining of substantial amounts of powereconomically becomes more and more difficult. A practicable frequency,in the particular example described herein is 840 cycles. However, it isdesirable to have the advantages of still higher frequencies, and thisis obtained to a considerable extent by superposing high frequencyharmonics upon the fundamental 840" cycle wave. These high frequencyharmonics have been obtained in the present instance by the shaping andslotting of the pole faces 'as shown in FIGURE 4, by eliminating theskew in the stator structure customarily used, and by utilizing awinding as shown in FIGURE 5. The use of these expedients singly or incombination, will yield a wave form with a high percentage of harmonicsas shown in FIGURE 11, FIGURE 11 (a) representing the oscillogram of thewave form at light load and FIGURE 11 (b), the wave form at substantialload.

It has 'also been found that the conducting or amortisseur bars placedcustomarily in the pole faces 20, may be omitted with a consequentincrease in the generated harmonic content.

FIGURE 5 shows 'the winding pattern used for the stator 6 of thegenerator. `The stator may, for example, have 84 slots. The windingpattern for each of the independent phases is shown. It will be seenthat each coil spans three slots and that the coil sides for adjacentcoils lie in the same slot. The phases are displaced by one tooth. Thewindings for each of the three phases are identical. A tap is broughtout from the fourteenth coil of each phase. This is for the neutral orground connection 26 mentioned herein. Except for the common groundneutral connection, the phases are electrically independent.

Use of the winding described has been found to yield a large percentageof odd harmonics, particularly the third 5 harmonic. This isadvantageous, as further detailed herein.

The connection diagram for the generator is shown in FIGURE 6.

FIGURE 7 shows the wiring diagram of the generator a-s presented to theexternal load. It will be seen that three individual windings areprovided, each displaced in phase relationship from the other. Although3 phases are shown, any number of phases, either one or a plurality maybe utilized, without departing from the spirit of the invention. A3-phase arrangement is shown, because 3-phase generators are most usual.Each of the coils has terminals 25, 25 and a center tap 26 which isgrounded, thus connecting the center points of all coils to a commonterminal and to ground. The advantage of this is that no point in eitherthe generator or external circuit is more than half the total voltageabove ground potential. The voltage between terminals 28-28 may be, forexample, 400 volts or 600 volts. The voltage across terminals 27-27 and25 25 may have the same value as the voltage across terminals 28-28.Alternatively, where it is desired to use fluorescent lamps of differentlengths, each with different voltage requirements, the voltage betweenterminals 28--28` may for example, -be 600 volts and that between 27 27,for instance, 400 volts. Due to the structure of the generatorsdescribed herein, the application of load to one pair of terminals, forexample 28 28, will have relatively little effect on that produced byload on terminals of another phase, 27-27. The elimination of aregulator as further described herein, makes it possible to wind themachine to produce different voltages from each phase withoutinterference with each other.

As a still further alternate, a tap 29 may be placed in any of thewindings. The voltage then obtained may be either that -between eitherof terminals 28 and 29 or between the ground terminal such as 26 and theterminal 29. This feature is especially valuable where there are to beutilized only a few lamps employing a different voltage than that of themajority of lamps utilized in the system.

Alternatively,' a tap such as 29, may be utilized for monitoring orcontrol purposes or for the operation of auxiliary devices other thanlamps.

A typical circuit employing fluorescent lamps is shown in FIGURE 8. Hereterminals 25, 25 are the leads from one phase of the generator. A switch33 controls the flow of current in this circuit, and for the sake ofgenerality a choke 32, a condenser 31 and a resistance equivalent to thedissipative load of the larnp 30, are represented in series.

FIGURE 8 may be taken to represent not merely one larnp but a series oflamps and their equivalent circuit.

For the sake of economy, a capacitor ballast 31 is utilized to limit thecurrent flowing in'the larnp 30. The value of capacitor 31 will dependupon the frequency generated by the generator, the harmonics presenttherein, the voltage of the generator and the current that is desired toflow through the lamp 30'.

In order to effect economy in the wiring, switches and the Converterunit itself, it is desirable to correct or partially correct the leadingpower factor due to the larnp 30 and the capacitor 3-1. `The net powerfactor of this combination is usually of the value of 30% leading. Thechoke coil 32 may be placed in series with each individual capacitor,but is preferably used in series with or parallel across the circuitscomprised of a number of lamps and their respective capacitor ballasts31.

It has been found with the generator described, that the regulation;i.e, the change in voltage upon application of load can be brought to alow figure, very near zero, by adjusting the power factor presented tothe generator to a value of approximately .85 leading. With such powerfactor, due to the Construction of the generator and the' relativelyconstant rotational speed due to the induction motor operated from aconstant frequency supply line, the' voltage will remain very stable,not varying by more than about plus or minus 2%. This is achievedwithout the use of any auxiliary regulating equipment.

The preferred method of adjusting power factor in a circuit of the typeshown in FIGURE 8, is to adjust the value of choke 32 until a netleading power factor is obtained, corresponding to that figure for whichthe generator will have a minimum regulation or a regulation within thedesired voltage tolerance.

For example, plus or minus 5% the output voltage. It is then necessaryonly to correct the power factor to such a value that the net change involtage will be no greater than plus or minus 5%.

This procedure makes it possible to utilize a smaller choke than wouldotherwise be necessary, with consequent economy in first cost and inoperation. It also makes it possible to avoid entirely the correction ofcircuits containing only a few lamps, since the overall effect of theirload and power factor on the entire system, will be so small as to beneglected. The use of a wave with a high percentage of harmonics makesthe choke more effective than it would be for the fundamental frequencyalone and therefore permits a still further reduction in the size of thechoke.

S-till further features of Construction which make it possible to reducethe size of the choke, and in some cases to eliminate it altogether, aregiven below:

If the motor 2 in FIGURE 1 is chosen so as to show a great speed dropwith application of load, in other words, to have a high slip, if aninduction motor, the application of load will cause a reduction in speedof the generator 3, which will have the effect of reducing the terminalvoltage and also reducing the frequency. Both of these effects will tendto reduce the voltage applied across the circuit shown in FIGURE 8 andwill tend to reduce the voltage and current through larnp 30. However,the load imposed by the larnp 30 with its capacitor 31 is leading incharacter and a leading load of the magnitude and power factor notedabove, has the effect of raising the terminal voltage of the generator.Thus, the terminal voltage ncreasing effect and the terminal voltagedecreasing effect due to -slowing down of the motor and generator, willtend to oppose each other and to tend toward a constant voltagecondition. This, as noted above will make it possible to reduce the sizeof the choke and in some cases to eliminate it altogether.

A still further correcting circumstance occurs in the case of the waveform. The effect of the capacitive load of the type described, will beto suppress the harmonics and to leave a wave with the greaterproportion of the fundamental. Since the capacitor 31 will tend to passthe high frequency Components more readily than the lower frequency, theremoval of these high frequency components or the reduction of theirpercentage will cause a reduction in the net current flowing through thelarnp 30.

At the same time, the application of the leading load to the generatorwill tend to cause an increase in the terminal voltage of the generator.This tendency to increase voltage and thus increase the current throughthe lamp 30 will at the 'same time be counterbalanced, at least in part,by the reduction in the harmonics as noted above.

Any of the expedients noted above tending toward the maintenance of theconstant voltage condition may be used singly or in any combination.They will in all cases, tend toward achieving the ideal of a constantcurrent through larnp 30, regardless of the total load in the system.

Where a direct current motor is utilized for the driving means in FIGURE1, a governor may be utilized on the shaft of the motor, or other speedsensitive devices may be utilized, in order to maintain relativelyconstant speed. Alternatively, the speed control device may be selectedso as to have a droop suitable for correction of the voltage may betolerated in increase which tend to be caused by the application of theleading load on the generator.

The rotating system composed of rotor 7 of the gen erator and -rot-or ofthe motor on shaft 8, will have a high moment of inertia, particularlysince the rotor 7 is in the shape of a fly wheel. This will tend to givethe converter a stabilizing efect and to resist the efiects of shortperiod disturbances on the input supply lines. The latter are oftencaused by switching on or off of large motors. i

Since with the usual type of induction motor, the input voltage hasrelatively little elfect on the Operating speed, the converter as shownwill be quite insensitive to voltage changes on the input supply line.The output voltage will, in general, tend to remain more constant thanthe input voltage, so that a high quality of lamp operation may beobtained from a low quality of supply.

The use of the correction systems noted above will avoid the use ofcomplex and delicate regulating auxiliaries. Since the converterdescribed is intended for use in localities where skilled personnel arenot ordinarily available for maintenance of delicate and complexequipment, the elimination of such equipment and the necessity thereforis an important step in the more widespread use of the high frequencysystems.

Where alternating current is utilized for the drive of the motor 2, nobrushes Will be required in the motor and since the generator portionlikewise does not utilize brushes, the installation may be madeexplosion-resistant or explosion-proof.

A plurality of converter units as shown in FIGURE 1 may have theiroutputs connected in parallel. For such purpose, the internal impedancesof the generator portion of the machines are made nearly identical andthe terminal voltage for a given condition of load are made nearlyidentical. The machines may then be interconnected in parallel so as toshare the load when the latter becomes too great for one machine tohandle alone.

As an alternate, the motors 2 may be synchronous motors, particularlybrushless types of synchronous motors. If the stators 6 of thegenerators are all arranged so as to bear the same angular relation tothe stator 4 of t eir respective motors, the outputs of the generatorsmay then 'be connected in parallel, since they will always be in thesame phase relationship and will remain so due to the constant speedcharacteristics of the synchronous motors.

As will be obvious to those skilled in the art, the features ofConstruction of the converter units may be applied singly or in anycombination or may be applied to purposes other than the operation ofhigh frequency fluorescent lighting systems.

The principles of matching of load and converter or load and generatormay be applied to loads other than those due to fluorescent lamp andtheir auxiliaries.

The scope of the invention is indicated by the appended elaims.

What is claimed is: v

1. In a frequency converter for fluorescent lighting applications, anelectric driving motor and a generator coupled thereto so as to memechanically energized thereby, a permanent magnet rotor of agenerally'flywheel shape, having a diameter large compared to its`length in said generator and a plurality of armature windings withinsaid generator concentric with and surrounding said rotor, each windingbeing electrically independent from the others except for a common tapat the electrical midpoint of each winding, said midpoint taps beinginterconnected with one another.

2. In a frequency converter for fluorescent lighting applications, anelectric dn'ving motor and a generator coupled thereto so as to bemechanically energzed thereby, a permanent magnet rotor of a generallyflywheel shape, having a diameter large compared to its length in saidgenerator, said rotor including a plurality of uniformly spaced radiallydisposed permanent magnets, with their magnetic axes passing through thecenter of rotation thereof and a generator stator surrounding saidrotor, and both conductively and inductively independent of said drivingmotor and of the source of energy applied to said motor.

3. In a frequency converter for fluorescent lighting applications, aunitary two bearing assemblage consisting of a housing, a motor statorand a generator stator, within said housing, a shat, a motor rotor uponsaid shaft and a generator rotor also mounted upon said shaft, saidmotor and generator rotors being rigidly attached to said shaft so as torotate therewith, the generator having a greater number of poles thanthe motor so as to generate a higher fundamental frequency than thatsupplied to the motor, a generator stator surrounding said generatorrotor electrically and inductively independent of the motor stator andthe electric current supplied to said motor, and means within saidgenerator to generate harmonics, said means including a stator structurewith teeth substantially parallel to the axis of rotation of the rotorand said rotor includes pole pieces With sharply defined edges parallelto the axis of rotation of said rotor.

4. lu a frequency converter for fluorescent lighting applications, aunitary two bearing assemblage consisting of a housing, a motor statorand a generator stator within said housing, a shaft, a motor rotor uponsaid shaft and a generator rotor also mounted upon said shaft, saidmotor and generator rotors being rigidly attached to said shaft so as torotate therewith, the generator having a greater number of poles thanthe motor so as to generate a higher fundamental frequency than thatsupplied to the motor, and a generator stator surrounding said generatorrotor electrically and inductively independent of the motor stator andof the electric current supplied to said motor, said stator including aWinding containing conductors in each coil spaced apart a sufiicientnumber of electrical degrees to generate a substantial third harmonicfrequency of the fundamental generated frequency.

5. In a frequency converter for uorescent lighting applications analternating current driving motor and an alternating current generatorcoupled thereto, housing means enclosing both the motor and generatorportions, a unitary shaft member within the housing carrying both themotor and the generator rotor members, at least one bearing member beinglocated intermediate the motor rotor and the generator rotor, thegenerator rotor being mounted exterior of said bearing and having noother bearings associated therewith, the generator rotor being of apermanent magnet type and having a number of poles substantially greaterthan the number of poles in the motor stator structure, said generatordelivering an output' frequency of substantially 840 cycles and havingmeans provided within the rotor and stator thereof respectively, tosuperpose Odd' harmonics of 840 cycles upon the output wave.

6. In a frequency converter for fluorescent lighting applications analternating current driving' motor and an alternating current generatorcoupled thereto, housing means enclosing both the motor and generatorportions, a unitary shaft member within the housing carrying both themotor and' the generator rotor members, at least one bearingmember beinglocated intermediate the motor rotor and the generator rotor, thegenerator rotor being mounted exterior of said bearing and having noother bearings associated therewith, the generator rotor being of a`permanent magnet type and having a number of poles substantially greaterthan the number of poles in the motor stator structure, said generatordelivering a generated frequency of 840 cycles and having means withinthe rotor and stator thereof respectively to generate odd harmonics andsuperpose them upon the generated 840 cycle output frequency, saidstator further having polyphase windings ncorporated therein.

References Cited by the Examie- UNITED STATES PATENTS 1 O FOREIGNPATENTS 578,934 7/ 1946 Great Britain. 599,730 3/ 1948 Great Britain.626,611 7/ 1949 Great Britain.

LLOYD MCCOLLUM, Primary Examiner.

4. IN A FREQUENCY CONVERTER FOR FLUORESCENT LIGHTING APPLICATIONS, AUNITARY TWO BEARING ASSEMBLY CONSISTING OF A HOUSING, A MOTOR STATOR ANDA GENERATOR STATOR WITHIN SAID HOUSING, A SHAFT, A MOTOR ROTOR UPON SAIDSHAFT AND A GENERATOR ROTOR ALSO MOUNTED UPON SAID SHAFT, SAID MOTOR ANDGENERATOR ROTORS BEING RIGIDLY ATTACHED TO SAID SHAFT SO AS TO ROTATETHEREWITH, THE GENERATOR HAVING A GREATER NUMBER OF POLES THAN THE MOTORSO AS TO GENERATE A HIGHER FUNDAMENTAL FREQUENCY THAN THAT SUPPLIED TOTHE